The invention relates to a backscatter transponder for generating an oscillator signal based on a base signal with an oscillator for actively constructing the oscillator signal by means of oscillations, an input for the base signal and an output for the oscillator signal generated, whereby the oscillator is rendered capable of being activated in a quasi-phase-coherent manner with the aid of a control signal generated by a clock generator and is capable of being activated in a quasi-phase-coherent manner with respect to the base signal by means of the base signal for generating the oscillator signal.
The invention also relates to a communication system incorporating such a backscatter transponder and/or a method for transmitting data with such a backscatter transponder.
Methods and arrangements for exchanging data and for measuring the distance from a base station to a modulated transponder exist in numerous forms and have been known for a long time. Customary transponders comprise what are referred to as “backscatter transponders”, for example, which do not have their own signal source but instead simply reflect back the received signal (where relevant, in amplified form). Reference is also made in this context to “modulated backscatter”.
Although the backscatter transponder constitutes the data transmitter, a dedicated radio frequency signal is not customarily generated in a backscatter transponder. A radio frequency auxiliary carrier signal is sent first from the actual data receiver station to the transponder, which this transponder sends back, usually with low-frequency modulation.
The critical advantage of communication systems based on backscatter transponders with respect to standard communication systems having separate signal sources in all sub-stations therefore consists in the fact that the signal received in the receiver can be restricted to the modulation bandwidth in a virtually optimal manner via mixing with the auxiliary carrier signal and therefore a virtually optimal signal-to-noise ratio is achieved.
With the separate signal sources in the transmitter and receiver which are otherwise customary in communication systems, it is generally not possible, or only with great effort, particularly in the case of lower data rates, to regulate the separate sources in such a precise manner with respect to frequency and phase that a comparably small receiver bandwidth would be achievable.
The critical disadvantage of backscatter transponder systems, however, is the fact that the radio frequency signal has to travel along the path from the receiver to the transponder and back and therefore, based on the radar equation, the signal-to-noise ratio (SNR) for the overall transmission link decreases in proportion to the 4th power of the distance. Due to free field attenuation which increases strongly with frequency, it is scarcely possible to implement very high-frequency backscatter transponders in the GHz range, particularly with a satisfactory signal-to-noise ratio.
If, as is customary in the case of standard communication systems, a data signal is generated in the data transmitter, particularly in the transponder, with a dedicated source, the RF signal travels along the transmitter/receiver path only once. In this case, the SNR is only inversely proportional to the square of the distance. Added to this is the fact that other attenuation/losses on the transmission path also only affect the signal once and not twice. Therefore, particularly in the case of larger distances, the SNR is orders of magnitude higher in this respect than in the case of simple backscatter systems.
A device for generating an oscillator signal based on a base signal having an oscillator for actively constructing the oscillator signal via oscillations, an input for the base signal and an output for the oscillator signal generated is known from German patent document DE 100 32 822 A1 in which the oscillator is capable of being activated by the base signal to generate the oscillator signal in a quasi-phase-coherent manner with respect to the base signal. In this respect, the device comprises particularly a transmitter in the form of a transponder and provides an oscillator connected to the transponder antenna. A clock control unit is additionally provided for activating the oscillator. The oscillator is switched on and off cyclically with the clock control unit via a clock control signal. In this respect, the signal generated by the oscillator is quasi-coherent with respect to the received base signal. Switching the oscillator on and off also switches its quasi-phase-coherent activation capability.